Hi Emma, the easiest way to surface that list of members is to go to your Manage Members area, and filter by "last visit" date. Choose a date possibly a few months ago, and you'll see who hasn't visited the site recently.
Once you have the filtered list, you can send out a communication using the "notify members" in the dropdown to say "hey, we miss you!"
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .